The present invention relates to a television transmission system in which the video components of a transmitted television signal are scrambled to allow only for their authorized reception, a video scrambling key being provided for the line-by-line scrambling or de-scrambling of said video components, said video scrambling key comprising a first pulse signal having a first pseudo random configuration generated at a first rate which corresponds to that of the line frequency, the first pulse signal being periodically set, in response to control data having a rate which is low compared with that of the line frequency, to a particular pulse configuration which first pulse signal thereafter is stepped in accordance with its random configuration. The invention also relates to transmission and receiver equipment for use with such a system.
A system of the above type is known where each line of the video components of a television signal is scrambled by cutting the line and changing the cut-point for each line. This is done in a random manner by a locally generated random video scrambling key at the transmission source and a corresponding random video scrambling key locally generated in the television receiver. Both scrambling keys are periodically reset by control data at a rate which is low relative to that of the line rate of the television signal, which control data is transmitted with the television signal. Such a method provides a relatively secure scrambling system for the video components. Providing the scrambling key for such a system line rate presents little or no problem in transmission equipment but a problem is present in a receiver if it incorporates a separate sub-system, which could be in the form of a card carrying an integrated circuit capable of being inserted into an appropriate place of a television receiver. The problem is that data at a high data rate (up to 20.25M bits/second in the case of a C-MAC television signal) would need to be conveyed between the card and the receiver and vice versa which is not as practical as might first appear.